Automatically adjustable power supply for x-ray tubes



United States Patent 3,183,357 AUTOMATICALLY ADJUSTABLE PDWER SUPPLY FORX-RAY TUBES William J. Phelan, Olyphant, and Iiames B. MacPherson,

Clarks Summit, Pa., assignors to Weston Instruments,

Inc., a corporation of Texas Filed Gct. 22, 1962, Ser. No. 232,676 4Claims. (Cl. 250-103) The present invention relates to an accuratelycontrolled adjustable power supply for an X-ray tube and has for anobject the provision of a system of compensation for undesirablevariations in X-ray tube beam intensity which occur when the supplylevel setting is changed.

Adjustable high voltage power supplies for X-ray tubes have heretoforebeen used to set the intensity of the X-ray beam at difien'ng desiredlevels. However, a disadvantage in these systems is that the beamintensity will not vary directly with change in setting of the supplypotential as a result of static charge which has been built up on thewalls of the X-ray tube. For example, upon change from a higher levelsetting to a new lower level setting the excess of the static chargebetween the two levels produces, before it has leaked oti, values oftube resistance, tube potential and tube current different from theirsteady state values ordinarily produced at the new setting. As theexcess charge leaks olf, the foregoing values will change and thus thebeam intensity will vary until the new steady state values have beenreached. This variation in beam intensity for each new setting is veryundesirable when there is required rapid change between settings alongwith a high degree of accuracy.

Accordingly, an object of the present invention is to rapidly compensatefor undesirable variations in X-ray beam intensity resulting from theleaking off of excess static charge which has been built up in the tube.

Another object of the invention is to compensate for undesirablevariations in intensity with the compensation being a function of thevariation of the average tube current.

In a preferred form of the invention there is connected in circuit withthe X-ray tube, means for producing a compensation signal which variesas a function of the current flow through the X-ray tube. In response tothe magnitude of this compensation signal, the high voltage applied tothe X-ray tube is adjusted to compensate for the undesirable variationsin beam intensity.

In carrying out the invention in one form thereof a source of supply isconnected by way of a high voltage step-up transformer to the X-raytube. A control circuit is connected to the source for comparing theoutput thereof with a selected reference potential level. As a result ofthis comparison, there is produced a feedback to the source to controlthe magnitude of the high voltage applied to the X-ray tube. Inaccordance with the present invention the compensation signal whichvaries as a function of the tube current is applied to the controlcircuit for further changing the feedback. As a result, the magnitude ofthe high voltage applied to the X-ray tube is varied as a function ofthe changing tube current to provide the compensation for theundesirable variations in beam intensity. In this manner, and inaccordance with the above example, upon change from a high to a lowerlevel setting of supply potential, the X-ray tube beam intensity isconstantly and accurately maintained at its desired new steady statevalue.

Further, in accordance with the invention, the direction of compensationfor variation in beam intensity is dependent on the efiective resistanceof the X-ray tube with respect to the effective source resistance of thepower supply. Thus, for example, upon change to a new lower levelsetting of the supply, the tube resistance and the tube potential are ata higher value and the tube current is at a lower value than their newsteady state values. Since the resultant beam intensity is proportionalto the square of the tube potential times its current, if the tuberesistance is less than two times the source resistance, the intensitywill have been increased beyond its steady state value. To compensatefor that increase, the tube potential is decreased accordingly. On theother hand, if the tube resistance is greater than two times the sourceresistance, the resultant intensity will have been decreased beyond itssteady state value. To compensate for that decrease, the tube potentialis increased accordingly.

For further objects and advantages of the invention and for adescription of its operation, reference is to be had to the followingdetailed description taken in conjunction with the accompanying drawingsin which:

FIG. 1 schematically illustrates an adjustable power supply for an X-raytube according to the invention with the X-ray tube resistance beingless than two times the source resistance; and

FIG. 2 schematically illustrates a further embodiment of the inventionwith the X-ray tube resistance being more than two times the sourceresistance.

Referring now to FIG. 1, the adjustable power supply for the X-ray tube10 has been illustrated as including a high voltage step-up transformer11 having the tube 10 connected to its secondary windings 11b and 11c.The primary winding 11a of transformer 11 is connected to a source ofsupply including a transformer 13, a diode 15, and a control circuit 16.A regulated source 14 applies an alternating current voltage to theprimary winding 13a of the stepup transformer 13 to produce at thesecondary winding 13b a regulated alternating current voltage. Thisvoltage is rectified by the diode 15 so that only the positive-goinghalf cycles are applied therethrough and to the plates 20a and Zllb oftube 20. The cathodes 20c and 20d of tube 20 are connected by way ofresistors 21 and 22, respectively, to a common junction and then by wayof ground to the lower end of primary winding 11a, the upper end ofwhich is connected by way of conductor 18 to the upper end of winding13b.

Tube 20 of control circuit 16 is maintained normally conductively biasedso that upon application thereto of a positive-going half cycle currentflow may be traced from winding 1312, through diode 15, tube 20, toground, and through winding 11a, conductor 18 to winding 13b. In thismanner, there is provided a low impedance path between the secondarywinding 13b of the source transformer 13 and the primary winding 11a ofthe high-voltage transformer 11 which is efiective to produce a highvoltage across its secondary windings 11b and 110.

In the secondary circuit of transformer 11 the upper end of winding 11bis connected to the anode of tube 10, one terminal of the filament ofwhich is connected to the lower end of winding 110. The upper end ofwinding 11a is connected by way of resistor 12 to ground. Thus, currentflows from winding 11b, through the X-ray tube 10, winding llc and thenby way of resistor 12 to ground. The resultant potential dropacross-resistor 12 is positive with respect to ground and isproportional to the peak values of the X-ray tube current. It will laterbe described how that potential drop is eifective to provide thecompensating action in accordance with the present invention.

When tube is non-conductively biased, the supply circuit for primaryWinding 11a during the positive half cycles may be traced from winding1312 by way of diode 15, resistor 23, ground, winding 11a, conductor 18and then to the winding 13b. Since the resistor 22 is selected to have asubstantially large value, the potential that is then produced acrossprimary winding 11a is negligible compared with its value when tube 20is conductive. Thus, the biasing of the tube 20 controls the amount ofthe half cycle that is effectively applied to the winding 11a and inthis manner increases or decreases the energizing high voltage appliedto the X-ray tube.

The controlled half cycle appearing at the upper end of winding 11a isof a negative-going potential and that upper end is connected by way ofconductors 18 and 23, diode 24, winding 26a, contact 28a andpotentiometer 28 to ground. The upper end of potentiometer 28 isconnected to a source of negative reference potential, as for examplebattery 30, and the setting of the slider 28a on the resistor 28determines the potential at the slider with respect to ground. Thus, thediode 24 is not rendered conductive until the leading edge of thenegative half cycle has reached a magnitude of greater value in anegative direction then the negative potential applied to the slider28a. At the time diode 24 is rendered conductive a signal is producedacross the primary 26a of the transformer 26 which results in an outputacross its secondary 25b.

As previously described, X-ray tube current flow through resistor 12 iseffective to produce at the upper end thereof, a potential drop positivewith respect to ground. This potential drop has the effect of reversebiasing diode 31 which reverse bias must be overcome by a negativepotential produced at the upper end of winding 26b with respect toground before diode 31 will become conductive. When that negativepotential is sufficient to render diode 31 conductive, current fiow inthe secondary circuit of transformer 26 may be traced from groundthrough resistor 33, conductive diode 31, winding 26!), resistor 12 toground. In this manner with diode 31 conductive, an input signal isapplied to the amplifier 32.

It will now be understood that the effect of the reverse bias positivepotential produced across resistor 12 is to increase the amount by whichthe output signal across winding 26b must exceed the level of the biasbefore diode 31 is rendered conductive and an input signal is applied toamplifier 32. It is in this manner that the desired compensating actionis produced by the proper selection of the resistance value of resistor12. The amplifier 32 is of the conventional resistance capacitancecoupled type and thus amplifies and inverts the input signal which isthen applied by way of conductor 34 to the input of a one-shot ormonostable multivibrator 37.

One-shot multivibrators are well known in the art and are described indetail in Pulse and Digital Circuits, by Millman and Taub, McGraw-Hill,1956, at page 174 et. seq. As set forth in this text a one-shotmultivibrator is induced to make a transition from its stable state toits quasi-stable state by the application of an input signal or pulse.After such transition, the multivibrator 37 will remain in itsquasi-stable state for a desired period of time by proper selection ofcircuit element values and connections as described in detail in theforegoing text. In the present invention, the multivibrator 37 has aquasi-stable state time duration at least equal in time to one-halfcycle of the frequency of the regulated source 14 to provide propertiming of the nonconductive bias to tube 20, as will later be explainedin detail. The output of the multivibrator 37 provides a positive-goingpulse of rectangular wave shape during the time of the stable state andupon transition to its quasi-stable state there is produced anegative-going pulse of rectangular wave shape.

The output of the multivibrator 37 is applied to D0. restorer 39comprising a capacitor 39a, a triode 39b, and a resistor 390.Accordingly, the output of the multivibrator 37 is connected by way ofcapacitor 39a to the common connection of the plate and grid of tube 3%and the resistor 390, the other end of which resistor is connected toground. D.C. restorers of similar type are described in detail at page119 et. seq. of the above-named teXt where it is shown that the top orpositive extremity of the positive-going pulses are restored or clampedto ground. Thus, during the time of the stable state the positive-goingoutput pulse from multivibrator 37 is clamped to ground potential whichis applied by way of conductor 42 and resistors 43 and 44 to therespective grids 29a and 20f of the tube 20. Accordingly, the resultantpotential applied to the grids 20a and 20 with respect to the cathodes20c and 20d is effective to maintain conductive tube 20 during the timeof the stable state of the multivibrator 37.

Upon application of a switching pulse to the multivibrator 37, theoutput negative-going pulse of rectangular wave shape achieves, by meansof the restorer 39, a negative potential with respect to ground. Thisnegative potential is applied as a feedback signal by way of conductor42 and resistors 43 and 44 to nonconductively-bias tube 20, thuseffectively to cause current not to flow through the winding 11a. Sincethe time duration of the quasi-stable state and thus the time durationof the negative pulse is at least one half cycle, the tube 20 ismaintained nonconductive at least until the termination of the negativehalf cycle produced across winding 13b.

It will now be understood that the resultant wave shape of the signalproduced across the winding 11a will be the wave shape of the leadingedge of the half cycle with the Wave shape returning to ground potentialat the time tube 26 is rendered nonconductive. By adjusting the settingof slider 28a, the time is changed during the leading edge when tube 20is rendered nonconductive and thus, to change the resultant magnitude ofthe pulse developed across winding 11a.

It will be assumed in this embodiment of the invention, that theresistance of the tube 10 is less than two times the effectiveresistance of the source which may be considered to be the resistancelooking back into the supply circuit from the tube 10. Accordingly, aspreviously described, upon change to a new lower level setting, the beamintensity increases beyond the value it would ordinarily have at thatnew setting. In order to provide compensation for that increase, thehigh voltage supply potential applied to the tube 10 must be decreasedaccordingly.

Thus, in operation of the adjustable power supply of FIG. 1, the slider28a may be moved in a downward direction to decrease the referencepotential applied thereto when it is desired to decrease the beamintensity setting to a new lower value. As a result of the excess staticcharge, the tube current decreases to a value lower than the steadystate value it would have ordinarily at the new setting of the slider28a. As a result of this decreased current flow, the potential dropacross resistor 12 is decreased in proportion to the peak tube currentso that the reverse bias on the diode 31 is also lower than its steadystate value at that new setting. Thus, a lower value of negativepotential is allowed to pass through diode 31 which is amplified byamplifier 32 to trigger multivibrator 37 to its quasi-stable state at apoint in time before it would ordinarily be triggered. In this mannerthe tube 20 is rendered nonconductive at an earlier time on the leadingedge of the half cycle thus to produce a pulse across the winding 11a ofmagnitude less than the value it would ordinarily have at the newsetting. In this manner, the high voltage supply potential for the tube10 is decreased as a function of the changing peak tube current toprovide the compensating action for the increased beam intensity. Thatdecrease in potential will exactly balance out the increase in intensityby proper selection of the resistance value of resistor 12.

As the excess static charge leaks off from the walls of the X-ray tube10, the tube current will approach its steady state value which will beaccurately and rapidly followed by the compensating action to provide atthe new setting a constant value of beam intensity.

Referring now to FIG. 2, it will be seen that many of the parts of theadjustable power supply for the X-ray tube are identical with those ofFIG. 1 and, therefore, have been identified by corresponding referencecharacters. However, it will be noted that the upper end of winding 110is connected by way of a parallel combination of resistor 12:! andcapacitor 47 to ground. The upper end of the parallel circuit isconnected by way of resistor 46 and capacitor 49 to ground. Accordingly,resistors 12a and 46 and capacitors 47 and 49 comprise a filter circuitto provide a potential across the capacitor 4 which is proportional tothe average D.C. component level of the pulse signal applied from thewinding 110. The time constant for this filter circuit is selected to belong with respect to the frequency of the supply 14, but very short withrespect to the incremental changes of the setting of the slider 28a.

The upper end of capacitor 49 is connected to ground by way of a voltagedivider comprising resistors 48 and 12b. Resistor 12b is shunted bycapacitor 51 which is effective to absorb the pulse from winding 2612without loss in amplitude of the pulse. The common connection of theresistors 48 and 12b is coupled by way of resistor 33, diode 31,secondary winding 26b, and battery 45 to ground. The battery 45 isconnected to reverse bias diode 31 while the potential drop producedacross resistor 12b is efiective to produce a forward bias for thatdiode. Accordingly, the values of the foregoing components are selectedso that during the time the transformer 26 is not energized, the diode31 is nonconductive. When that transformer is energized and anegative-going signal is produced across winding 26b of sufiicientmagnitude, diode 31 is rendered conductive thereby to provide a signalinput to the amplifier 32.

In the embodiment of FIG. 2 it will be assumed that the resistance ofX-ray tube 10 is greater than two times the effective resistance of thepower supply source. Accordingly, as previously described, upon changeto a new lower level setting of the slider 28a, the tube current and thebeam intensity will have been decreased below the values that they wouldordinarily have at the new setting. In order to provide compensation forthe decrease in intensity, the supply potential applied to the tube 10must be increased in the manner now to be described.

Upon change to the new lower level setting of the slider 25a, the tubecurrent decreases and the potential drop across resistor 12a, as Well asthe potential across capacitor 49, also decreases. As a result of thevoltage divider action of resistors 48 and 12b, the potential at thecommon junction thereof is decreased as a proportion of the potentialacross capacitor 49 which is applied by way of resistor 33 to the anodeof diode 31 to decrease its forward bias to a value lower than itssteady state value at the new setting.

With the forward bias decreased, 3. higher value of negative-goingsignal is required to be produced across the secondary winding 26b inorder to render diode 31 conductive thereby to provide a signal input tothe amplifier 32. This higher valued negative-going signal is providedat a later time on the leading edge of the half cycle at which time themultivibrator 36 Will be triggered to render tube 20 nonconductive.Thus, a greater portion of the leading edge of the sine wave is appliedacross winding 110 which provides an effective pulse of magnitudegreater than the value it would ordinarily have at the new setting. Itis in this manner that the high voltage supply potential for the X-raytube 19 is increased to provide the compensation for the decreased beamintensity when the tube resistance is greater than two times the sourceresistance.

It will now be understood that the filter which comprises resistors 12aand 46, and capacitors 47 and 49 is effective to provide compensation asa function of the variation of the average tube current rather than as afunction of the peak tube current as provided in the system of FIG. 1.

Now that the principles of the invention have been explained, it will beunderstood that many modifications may be made. For example, the filterof FIG. 2 may be utilized with the system of FIG. 1 to providecompensation as a function of the variation of the average tube currentwhen, as assumed in FIG. 1, the tube resistance is smaller than twotimes the source resistance.

What is claimed is:

1. An adjustable power supply for an X-ray tube having compensation forundesirable variations in the intensity of the X-ray beam resulting fromthe leaking off of excess static charge built up in said tube when theapplied high voltage is decreased to a lower voltage comprising,

a high voltage source of supply,

a high voltage transformer having its primary winding connected to saidsource of supply and its secondary winding connected to said tube,

reference means for selectively producing reference potentials,

means connected to said source of supply and to said reference means forcomparing the output of said source with a selected reference potentialto produce a feedback signal,

means including a switching device coupled to said source of supply andto said primary winding and responsive to said feedback signal forcontrolling the magnitude of the voltage applied to said tube,

means connected in circuit with said tube for producing a potentialchange proportional to the change in the current flow through said tube,

and means responsive to the magnitude of said potential change forproducing a corresponding change in said feedback signal thereby tofurther change the magnitude of said voltage applied to said tube as afunction of the change in said tube current and to compensate for saidvariations in beam intensity resulting from the leaking oil? of saidexcess static charge.

2. An adjustable power supply for an X-ray tube having compensation forundesirable changes in beam intensity which occur when the applied highvoltage is decreased to a lower voltage comprising,

a high voltage source of supply,

a high voltage transformer having its secondary winding connected tosaid tube and its primary winding connected to said source of supply,

switching means connected in circuit with said source of supply and saidprimary winding and operable for changing the magnitude of the potentialproduced across said primary winding,

reference means for selectively producing reference potentials,

control means connected to said primary winding and to said referencemeans for comparing the magnitudes of the signals produced therefrom andto provide a feedback signal to said switching means,

impedance means connected in circuit with said tube for producing acompensation signal proportional to the change in the current flowthrough said tube,

and means responsive to the magnitude of said compensation signal andconnected to said control means for varying said feedback signal therebychanging said potential across said primary winding whereby the highvoltage for the Xray tube is varied as a function of the tube current toproduce compensation for said variations in beam intensity.

3. The adjustable power supply of claim 2 in which said responsive meansincludes at least one diode having an anode and a cathode and in which.there is provided means connecting said cathode to said impedance meansfor producing a reverse bias for said diode which varies as a functionof said compensation signal.

4. The adjustable power supply of claim 2 in which said responsive meansincludes at least one diode having an anode and a cathode and in whichthere is provided means connecting said anode to said impedance meansfor producing a forward bias for said diode which varies .as a functionof said compensation signal.

References Cited by the Examiner UNITED STATES PATENTS 1,953,889 4/34Mutscheller 25097 2,236,195 3/41 McKesson 250-403 2,726,356 12/55Rockafellow 323-22 X 2,792,503 5/57 Kiesel 250-103 2,810,838 10/57 Clappet al 250103 RALPH G. NILSON, Primary Examiner.

FREDERICK M. STRADER, Examiner.

1. AN ADJUSTABLE POWER SUPPLY FOR AN X-RAY TUBE HAVING COMPENSATION FORUNDESIRABLE VARIATIONS IN THE INTENSITY OF X-RAY BEAM RESULTING FROM THELEAKING OFF OF EXCESS STATIC CHARGE BUILT UP IN SAID TUBE WHEN THEAPPLIED HIGH VOLTAGE IS DECREASED TO A LOWER VOLTAGE COMPRISING, A HIGHVOLTAGE SOURCE OF SUPPLY, A HIGH VOLTAGE TRANSFORMER HAVING ITS PRIMARYWINDING CONNECTED TO SAID SOURCE OF SUPPLY AND ITS SECONDARY WINDINGCONNECTED TO SAID TUBE, REFERENCE MEANS FOR SELECTIVELY PRODUCINGREFERENCE POTENTIALS, MEANS CONNECTED TO SAID SOURCE OF SUPPLY AND TOSAID REFERENCE MENS FOR COMPARING THE OUTPUT OF SAID SOURCE WITH ASELECTED REFERENCE POTENTIAL TO PRODUCE A FEED BACK SIGNAL, MEANSINCLUDING A SWITCHING DEVICE COUPLED TO SAID SOURCE OF SUPPLY AND TOSAID PRIMARY WINDING AND RESPONSIVE TO SAID FEEDBACK SIGNAL FORCONTROLLING THE MAGNITUDE OF THE VOLTAGE APPLIED TO SAID TUBE,